I did some quick testing with `cjxl --progressive`, different quality levels, and https://google.github.io/attention-center/

A 1536w / 2048h jpg selfie: 244kb

Progressive JXL at quality=70: 151kb

Loaded 7kb of that file: looked okay up to approx 130px wide.

The first 18kb of that file was a suitable thumbnail up to approx 210px wide.

The first 50kb looked okay up to 300px wide.

The first 75kb looked okay up to 600px wide.

Maybe I need to do a lot more testing, but this seems to work alright?